Lithium secondary batteries incorporating organic electrolysis solutions have been widely used. They are advantageous in that they have high energy output per unit volume or unit weight in comparison with other batteries. In exploiting this advantage, researchers and engineers have been advancing the development for practical applications of the lithium secondary batteries as power sources for mobile communications devices, notebook-type personal computers, and electric cars.
To improve the performance of a lithium secondary battery, attempts have been made to use metallic lithium as the negative electrode. However, the repetition of charge and discharge causes dendritic metallic lithium to grow on the surface of the negative electrode. This may lead to an internal short circuit between the negative and positive electrodes, ultimately triggering explosion. To suppress the possibility of this dangerous situation, engineers have studied the formation of a thin sulfide-based inorganic solid electrolytic layer on the metallic lithium. An example of this study has been disclosed in the published Japanese patent application Tokukai 2000-340257.
The technology on the solid electrolyte for lithium batteries and other applications has been disclosed, for example, in “Solid State Ionics, 5 (1981) 663-666, ” “DENKI KAGAKU (Japanese expression meaning electrochemistry) 65, No. 11 (1997) 914-919,” the published Japanese patent application Tokukai 2001-250580, “J. Am. Ceram. Soc., 84 [2 ] 477-79 (2001),” the published Japanese patent application Tokukouhei 5-48582, the published Japanese patent application Tokukaihei 4-231346, “The 26th Symposium on Solid State Ionics in Japan, Extended Abstracts (2000) 174-175,” U.S. Pat. Nos. 6,025,094, and 5,314,765.
On the other hand, it has been revealed that when a thin sulfide-based inorganic solid electrolytic layer containing silicon sulfide is in contact with metallic lithium, the silicon in the silicon sulfide (SiS2) is reduced by the metallic lithium, and consequently the inorganic solid electrolyte degrades with time even at room temperature.
Generally, a compound layer having low ionic conductivity, such as an oxide layer, is formed on the surface of metallic lithium. When the oxide layer is formed, the reaction between the metallic lithium and the silicon sulfide tends to be suppressed. Therefore, if the oxide layer is removed to improve the performance of the battery, the inorganic solid electrolyte degrades distinctly with time due to the reaction between the metallic lithium and the silicon sulfide.